The present invention relates to a sputtering target used in forming a CoPt-base alloy that is a recording layer of a magnetic recording medium, a method of making the sputtering target, and a CoPt-base magnetic recording medium.
In recent years, the amount of information processed on computers has been increasing with the development of an information-oriented society and it has become necessary to record and reproduce information at higher densities than before.
At present, in hard disks that are the external storage devices of personal computers, a multilayer structure in which an underlayer and a recording layer are formed on a substrate made of an Al alloy, glass, etc. is the mainstream.
In recent magnetic recording media, it is necessary to improve the coercivity and the squareness and to reduce noise for performing recording at higher densities than before, and CoCrPt-base alloys represented by CoCrTaPt are mainstream. In the CoCrTaPt, Pt which is effective in increasing the coercivity is added to conventionally used CoCr-base alloys.
The above recording layer is formed by the magnetron sputtering method through the use of an alloy target. When magnetron sputtering is performed through the use of a ferromagnetic target made of a Co alloy, the magnetic flux from magnets disposed at the bottom side of the target does not easily leak to the top surface of the target, causing plasmas to occur locally and the target to be partially consumed. For this reason, the problem of, for example, shorter target life arises. For improving this problem, reducing the magnetic permeability of materials for targets is effective, and various methods have been proposed.
For example, JP-A-2-49384 discloses a method for raising the target life by the improvement of magnetic properties, which improvement is achieved by applying strains to the interior of a Co alloy through cold working to thereby change the crystal structure of the alloy. JP-A-8-25270 discloses a method of reducing the magnetic permeability of a Co-base alloy target, which reducing is achieved by heat treatment performed after rolling etc. so that the crystal structure may be changed while precipitating intermetallic compound phases at grain boundaries.
Further, there is another problem that pass through magnetic flux is disturbed due to a nonuniform structure of the target, making the magnetic properties of the film nonuniform. Especially in the case of alloys containing large amounts of elements with different atomic weights represented by a CoCrPtTa alloy target, casting segregation is apt to occur during the melting and casting steps, and besides plastic workability becomes worse due to the occurrence of intermetallic compounds, making it difficult to obtain a uniform structure. For this reason, various methods as, stated below have been proposed.
JP-A-5-86456 discloses a method that comprises the steps of packing an ingot formed by melting and casting, performing HIP (hot isostatic pressing) after the heat treatment of the ingot and then performing rolling. JP-A-5-247638 discloses that it is possible to obtain a uniform structure and to lower magnetic permeability by the steps of melting and casting an alloy containing Co as the main component, 5 to 20 atomic % Cr, 10 to 55 atomic % Pt, 1 to 15 atomic % in total of both Ni and Ta, and 10 to 1500 ppm rare earth metals, forging, hot rolling and cold rolling.
Further, JP-A-5-247641 discloses a method of making the structure of a target uniform by sintering alloy powder containing Co as the main component, 4 to 18 atomic % Cr etc., 0.5 to 16 atomic % Pt, 0.1 to 8 atomic % Nb, and Ta etc.
As mentioned above, most of the improvements hitherto made on Co-base alloy targets aimed at reducing magnetic permeability and making the structure of a target uniform in order to improve the target use efficiency.
Furthermore, JP-A-4-297572 proposed by the present applicant discloses dispersing Pt particles in a sintered structure in order to increase the toughness of materials for targets. This technique can improve brittleness in a case where Pt is completely alloyed.
The present inventors are mainly engaged in the research and development of targets for sputtering devices of a stationary, opposing type. In this type, films are formed by means of a circular substrate disposed in a position opposed to a circular cathode, which is a typical structure of magnetic disk manufacturing device.
In this research and development, during the evaluation of the magnetic properties of films formed by means of CoPt-base alloy targets fabricated by the present inventors by a melting and casting process or a powder sintering process, the inventors faced a new problem that the coercivity of films changes continuously in the radial direction of the substrate. This is a major problem that causes a serious influence on the recording and reproducing characteristics of magnetic disks.
The present invention was made in view of the foregoing problem and an object of the present invention is to provide a CoPt-base sputtering target capable of obtaining a magnetic recording medium having a recording layer with uniform film characteristics and excellent recording and reproducing characteristics, a method of making the sputtering target, and a magnetic recording film formed by means of this sputtering target, and a CoPt-base magnetic recording medium in which this recording film is used.
As a result of elaborate examination of the above problem, the present inventors have found out that the chemical composition of a film changes continuously in the radial direction of substrate, that the amount of Pt in the film increases relatively with increasing distance from the erosion portion of the target, and that the change in the coercivity of the above film corresponds to the change in the amount of Pt in the film. In some specifications of the cathode of a sputtering device, inversely, the amount of Pt in the film decreases in the peripheral part of the substrate and it becomes also evident that the tendency of the change in the chemical composition of film differs from one sputtering device to another.
The present inventors made further elaborate examination of the structure of a target and the change in the film characteristics. As a result, they have found out that the variation of element distribution of the above film obtained with a target composed of an alloy phase occur due to a nonuniform distribution of elements which are sputtered from the alloy phase. Furthermore, the present inventors have found out that the element distribution of a film can be made uniform irrespective of the specifications of the cathode of a sputtering device by ensuring that a target has a structure in which the elements of the target are as little alloyed as possible and in which the elements are dispersed.
Finally, the present inventors have reached the present invention by finding out that in the case of a target made of a CoPt-base alloy containing elements other than Co and Pt, when at least Pt phase among the elements composing the target is finely dispersed while the thickness of a diffusion layer present at the boundary between the Pt phase and another phase is limited to be less than a predetermined value, it is possible to reduce the magnetic permeability of the material for the target and thereby to raise the use efficiency of the target while making uniform the Pt content within the film plane which Pt content has an especially great influence on the coercivity of the film.
The present invention provides a CoPt-base sputtering target containing Co as the main component, Pt as an indispensable element, and at least one kind selected from the group consisting of the 4a group elements, 5a group elements, 6a group elements, B and C. In the structure of this sputtering target, the diameter of a maximum inscribed circle of a phase consisting essentially of a Pt simple substance is substantially not more than 500 xcexcm, and the thickness of a diffusion layer at the boundary of the Pt phase is substantially not more than 50 xcexcm.
The target of the present invention may have a structure in which the elements composing the target are bonded to each other as a simple-substance phase.
Furthermore, the target of the present invention may have a structure in which a part of or all of Co is bonded as an alloy phase with at least a part of the elements other than Pt.
The present invention may be applied, for example, to a chemical composition consisting essentially of 0.1 to 25 atomic % Cr, 0.1 to 20 atomic % Pt, 0.1 to 15 atomic % Ta, and the balance Co and incidental impurities.
The above target of the present invention can be obtained by the steps of weighing and mixing Pt powder and other element powder selected from the group consisting of Co, 4a group elements, 5a group elements, 6a group elements, B and C, and sintering a resultant mixed powders at a temperature of 400 to 1000xc2x0 C. In this case, simple substance powder or alloy powder or their mixture can be used as the other element powder.
Magnetic recording films can be produced by sputtering the target of the present invention. Such magnetic recording films have a uniform Pt distribution in which a difference in the analytical value of Pt content measured in the radial direction of a disk is not more than xc2x110%.
Magnetic recording media such as hard disks can be obtained by using these thin films.